This invention relates generally to aircraft and their component systems, and, more particularly, to improved high-performance aircraft systems capable of high-altitude stationkeeping within tight altitude and perimeter boundaries for extended periods of time.
A worldwide expansion in the demand for communication bandwidth is driving up the bandwidth requirements between satellites and ground-stations. One way to increase this satellite-to-ground bandwidth is to interpose one or more high-altitude platforms (HAPs) configured for relaying signals between the two. A HAP allows for lower power transmissions, narrower beamwidths, as well as a variety of other advantages that provide for greater bandwidth. However, due to a demanding set of design requirements, years of design efforts at creating highly effective HAPs are only now beginning to come to fruition.
In particular, it is desirable to have a stratospheric aircraft, capable of carrying a significant communications payload (e.g., a payload of more than 100 kg that consumes more than 1 kw of electric power), that can remain aloft for days, weeks or even months at a time. This flight capability will preferably be maintainable even in zero or minimum sunlight conditions where solar power sources have little functionality. Also, the aircraft is preferably remotely pilotable to limit its weight and maximize its flight duration.
The communications payload preferably is configured to view downward over a wide, preferably unobstructed field of view. The aircraft will preferably be capable of relatively high-speed flight that is adequate to travel between its station and remote sites for takeoff and/or landing to take advantage of benign weather conditions. At the same time, the aircraft preferably is capable of maintaining a tight, high-altitude station in both high-wind and calm conditions, thus requiring relatively high-speed and relatively low-speed flight, and a small turning radius while maintaining the payload's downward-looking (and preferably upward-looking for some embodiments) view. To meet these stringent design specifications, the performance of the aircraft's power system, flight control system and airframe configuration and are all preferably improved over prior practice.
Power Systems
Conventional aircraft are typically powered using aviation fuel, which is a petroleum-based fossil fuel. The prior art mentions the potential use of liquid hydrogen as a fuel for manned airliners and supersonic stratospheric flight. There is also 25-year-old prior art mentioning the possibility of using liquid hydrogen as fuel for a stratospheric blimp.
U.S. Pat. No. 5,810,284 (the '284 patent), which is incorporated herein by reference for all purposes, discloses an unmanned, solar-powered aircraft that significantly advanced the art in long-duration, stratospheric aircraft. It flies under solar power during the day, and stores up additional solar power in a regenerative fuel cell battery for use during the night to maintain its station. The fuel cell battery is a closed system containing the gaseous elements of hydrogen and oxygen that are dissociated from, and combined into, water.
The aircraft disclosed in the '284 patent is an unswept, span-loaded, flying wing having low weight and an extremely high aspect ratio. Multiple electric engines are spread along the wing, which is sectionalized to minimize torsion loads carried between the sections. Most or all of the sections contain a hollow spar that is used to contain the elements used by the fuel cell. Large fins extend downward from inner ends of the sections. The wings contain two-sided solar panels within transparent upper and lower surfaces to take maximum advantage of both direct and reflected light.
The above-described technologies cannot provide for long-duration, high-altitude flights with tight stationkeeping when the available solar power is highly limited.
Flight-Control Components
Various components are known for use in controlling flight. Each component has unique advantageous and disadvantageous characteristics.
Many present-day small aircraft and some sailplanes use simple flaps to increase camber and obtain higher lift coefficients, and hence, adequate lift at lower speeds. Such flaps are typically retracted or faired to reduce drag during high-speed flight, and also during turbulence to reduce the maximum G loads that the wing will then experience. An important characteristic of the use of flaps, or of the use of highly cambered airfoils designed for high lift, is that the extended flap or highly cambered airfoil provides the wing with a large negative pitching moment. This affects both overall vehicle stability and the wing's torsional twisting. Indeed, for high aspect-ratio wings, the twist at the wing's outer portions due to a negative pitching moment can pose severe structural and flight control problems.
Airliners use both leading edge slats and sophisticated flaps, such as slotted or Fowler flaps, to widen their speed range. Small planes employ slats that open automatically when needed. Hang gliders have employed flexible airfoil tightening to decrease camber for high-speed flight. Some work has been done with flexible flap material that unrolls and pulls back from the rear of the wing. Some aircraft feature wings characterized by a sweep that can be varied in flight, even turning the entire wing so that it is not perpendicular to the flight direction during high-speed flight.
For maintaining low-speed flight without stalling, large solid or porous surfaces that hingedly swing up from a wing top in low-speed flight to potentially stabilize vortices immediately behind them, are known. This might provide an increased lift coefficient before stall is reached. Various vortex generators and fences are used to delay the onset of a stall or to isolate the portion of a wing that is stalled. Furthermore, various stall warning/actuators allow aircraft to operate relatively close to their stall speed. Additionally, some combinations of airfoils and wing configurations feature gentle stalls and so the vehicle can be operated at the stall edge without abrupt drag increases or lift decreases during the onset of a stall. Experimental aircraft have even employed rotary devices to permit low-speed flight, with mechanisms that restrict rotary moment and decrease drag or potentially augment lift when at higher speeds the wing provides the main lift. Many of the above mechanisms provide this increased low-speed control at the expense of weight and reliability.
In some high-tech aircraft, highly-active control is used to maintain stable operation over a wide range of speeds and orientations. This emulates the flying characteristics of natural fliers that change wing and airfoil geometry. In aircraft, such systems are complex, potentially heavy, and expensive, as well as fault-intolerant.
Airframe Configuration
The requirements for wide speed range, low power, light weight, unimpeded communications platform view, simplicity, and reliability present significant tradeoff challenges. A highly cambered airfoil helps with lowering minimum flight speed, but is accompanied by a large negative pitching moment that impacts the aeroelastic effects of wing twist.
Furthermore, there is an inherent relationship between an aircraft's overall airframe geometry and the design of its airfoils and control surfaces. Typical aircraft offset negative (i.e., nose-down) pitching moments through the use of tail moments (i.e., vertical forces generated on the empennage with a moment arm being the distance from the wing to the empennage) or through the use of a canard in front of the wing that, for pitch stability, operates at a higher lift coefficient than the wing and stalls earlier. Tails mounted in the up-flow of wingtip vortices can be much smaller than tails positioned in the wing downwash, but there are structural difficulties in positioning a tail in the up-flow.
Commercial airliners address the high coefficient of lift (CL) requirements for landing and takeoff with a complex array of slats and flaps that are retracted during high-speed flight to lower drag and gust-load severity. A rigid wing structure, and pitch controllability from the tail's area and moment arm, permit this approach. However, this approach is contrary to the requirement that the present aircraft carry fuel adequate to last for extended periods of time, and still be economical.
The very special requirements and technological challenges for the aircraft of the present invention have not been met by existing aircraft designs. Accordingly, there has existed a definite need for a lightweight aircraft capable of both stationkeeping and flight over a wide range of speeds, that consumes low levels of power for an extended period of time, that supports a communications platform with a wide, unobstructed view, and that is characterized by simplicity and reliability. Embodiments of such an invention can serve as high altitude platforms. Embodiments of the present invention satisfy various combinations of these and other needs, and provide further related advantages.